What is the tech troubleshooting trick that saved you the most time or money

[FairDos]

Every regular forum user tends to have at least one troubleshooting trick they picked up somewhere along the way that ended up saving them either a genuinely expensive repair, a callout fee, or hours of frustration once they finally figured it out, whether that is a simple restart that fixes a surprising number of issues, a specific setting buried deep in a menu that solved a persistent problem, or a slightly less obvious fix for a common hardware quirk that most people never think to try.

What is interesting about these tricks is how often they get discovered purely by accident or passed along informally from a friend, rather than from any official support documentation, and how often the eventual fix turns out to be embarrassingly simple compared to how long the actual problem was allowed to persist beforehand. So what is the troubleshooting trick that has genuinely saved you the most time, money or frustration over the years?